You should be able to check the pan by sealing the drain, and partially filling the shower pan, then see if the water disappears. Short of ripping up the tile I have no idea of the best way to stop the leak. Another thought, it could be a damaged drain pipe, and an inspection camera might show the crack or damage, any of the larger plumbers should be able to do this.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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